KINETICS OF CHROMOSOME CONDENSATION IN THE PRESENCE OF TOPOISOMERASES- A PHANTOM CHAIN MODEL

We discuss the requirement of type II DNA topoisomerase in the process of mitotic chromosome condensation. Using a known model describing th e collapse of homopolymers, we propose that the compaction process nec essitates a change in the topological state (i.e., a self-knotting) of the chromosomal chain. We argue that the enzymes are necessary to rea ch the compact metaphase state in a time interval that is much smaller than the time expected in the uncatalyzed process. The folding proces s is such that the potential entanglement points are localized at part icular regions of the chromosome known as the scaffold-associated regi ons. The concentration of entanglements in the metaphase chromosome is related to the average size of the radial loops. A phantom chain mode l for the condensation process, in which each potential entanglement p oint is dealt with by a topoisomerase II molecule, is proposed.